Twins are a kind of multiple births, i.e., when a woman gives birth to more than one baby at the same time. Twins occur when more than one egg is fertilized or when the same egg is fertilized more than once by one or two sperm, leading to formation of more than one fetus.
Conjoined twins are identical twins who are joined together somewhere in the body. Conjoined twins are monozygotic twins, in the sense that they share the same zygote. Sometimes, they also share some vital internal organs. Formation of conjoined twins is believed to be the result of late twinning. When the twinning occurs more than twelve days after fertilization of the egg, it may lead to formation of conjoined twins because the embryo may not split completely. This may be due to genetic or environmental factors.